1. Field of the Invention
The present invention relates to manufacturing small dimension features of objects, such as integrated circuits, using photolithographic masks. More particularly, the present invention relates to phase shift masking of complex layouts for integrated circuits and similar objects.
2. Description of Related Art
Phase shift masking has been applied to create small dimension features in integrated circuits. Typically the features have been limited to selected elements of the design, which have a small, critical dimension. See, for example, U.S. Pat. No. 5,766,806.
Although manufacturing of small dimension features in integrated circuits has resulted in improved speed and performance, it is desirable to apply phase shift masking more extensively in the manufacturing of such devices. However, the extension of phase shift masking to more complex designs results in a large increase in the complexity of the mask layout problem. For example, when laying out phase shift windows on dense designs, phase conflicts will occur. One type of phase conflict is a location in the layout at which two phase shift windows having the same phase are laid out in proximity to a feature to be exposed by the masks, such as by overlapping of the phase shift windows intended for implementation of adjacent lines in the exposure pattern. If the phase shift windows have the same phase, then they do not result in the optical interference necessary to create the desired feature. Thus, it is necessary to prevent inadvertent layout of phase shift windows in phase conflict near features to be formed in the layer defined by the mask.
In the design of a single integrated circuit, millions of features may be laid out. The burden on data processing resources for iterative operations over such large numbers of features can be huge, and in some cases makes the iterative operation impractical. The layout of phase shift windows and the assignment phase shift values to such windows, for circuits in which a significant amount of the layout is accomplished by phase shifting, is one such iterative operation which has been impractical using prior art techniques.
Because of these and other complexities, implementation of a phase shift masking technology for complex designs will require improvements in the approach to the design of phase shift masks.
A method for defining a full phase layout for defining a layer of material in an integrated circuit (IC) is described. In a full phase layout substantially all features of a layer of material, e.g. the polysilicon layer, are defined using phase shifting. By defining features using phase shifting, the majority of the layer can be composed of sub-wavelength features. For example if a xcex=193 nm stepper is used then the a feature significantly less than xcex in size is difficult to manufacture on the final IC without using phase shifting. By providing a systematic approach to placing, shaping, and assigning phase to the phase shifters, the method can produce high quality layouts that can be produced as photolithographic masks. Those masks can in turn be used in the production of a layer of an IC.
For a given pattern, e.g. the polysilicon (or gate) layer of an integrated circuit, the features can be identified. By growing a region around the featuresxe2x80x94except for end caps of featuresxe2x80x94a maximum shifter area can be defined. The maximum shifter area corresponds to the space where the shifters are desirably placed to define the features. Shifter shapes can then be placed against the edges of the feature. The shifter shapes are spaced apart from one another to leave open spaces where cuts, or openings, between different shifters may be necessary. The spacing requirement is related to the design rules for minimum spacing and edge length and may be different for different types of situations, e.g. outer and inner corner.
In some embodiments, the shifter shapes are a trapezoid stacked on top of a rectangle. This shape is designed to admit a cut that has a square notch at the top. Thus avoiding pointed corners which may be difficult to manufacture in a mask.
In some embodiments, the shifter shapes are then refined to fill certain open areas within the maximum shifter area.
Next, phase dependencies between the different shifter shapes are determined along with costs. This is important because there are certain requirements for an alternating aperture phase shifting mask, e.g. shifter on opposite sides of a feature should have opposite phase. However, there may be additional considerations beyond phase conflicts that should be considered. For example, how desirable, or undesirable, is it to have two shifters be the same phase on an inside corner, outside corner, along three edges, etc. Other criterion may include multiple-layer dependencies, e.g. positioning based on contacts, diffusion areas, etc. As well as cost functions for small shifters. Overall, the cost functions describe the relative quality of a given arrangement, e.g. shifter shape A and shifter shape B given same phase.
Phase can then be assigned to the shifter shapes according to the dependencies and the cost functions. After that, same phase shifters can be merged together filling some of the previously open cut spaces. Additional refinements are provided by some embodiments of the invention including removal of small shifters, squaring of corners, and filling open spaces with the dominant or subordinate phase.
After the phase shifters are defined, the trim shapes can be defined using the phase shifter shapes and the original pattern. In some embodiments, the logical or of the finished phase shifter shapes and the original layout are combined, down-sized to account for mask misalignment errors and then another logical or is performed with the original layout. The trim layout may include attenuated phase shifting shapes, e.g. tri-tone mask, etc.
In some embodiments, the input is a file containing the layout in a format such as GDS-II stream format and the output may be one or more files. In one embodiment, the output is a single GDS-II stream format file containing both the trim and phase layers. In other embodiments, the output is two mask data files, one for each mask, suitable for use by mask fabrication machines.
Embodiments of the invention include photolithographic masks. The photolithographic masks include a phase mask and a complimentary mask. The phase mask comprises a dark field, alternating aperture phase mask where the phase windows have been arranged to define the target pattern as described above. The complimentary mask comprises a mask designed to clear artifact left by the phase mask and define and remaining edges or edge segments not defined by the phase mask.
Embodiments of the invention include methods for manufacturing integrated circuits. The method includes exposing a layer of material in an IC using masks prepared and defined as described above.